DTC 85 Undefined Gear Ratio. GMC Truck GMC P42 Commercial Chassis 1998

For 1990-2009 cars only

Makes 🢒 GMC_Truck 🢒 1998 🢒 GMC P42 Commercial Chassis 🢒 DTC 🢒 DTC 85 Undefined Gear Ratio

Table 1:	4L80-E Range Reference, modified for DTCs P1870 & P1871
Table 2:	DTC 85

4L80-E Range Reference, modified for DTCs P1870 & P1871

Range	Gear	1-2 Sol	2-3 Sol	Fourth Clutch	Overrun Clutch	O/D Roller Clutch	FWD Clutch	Direct Clutch	Front Band	Interm. Sprag Clutch	Interm Clutch	Lo Roller Clutch	Rear Band
Overdrive	1st	On	Off	--	--	H	A	--	--	*	--	H	--
	2nd	Off	Off	--	--	H	A	--	--	H	A	O	--
	3rd	Off	On	--	--	H	A	A	--	O	A	O	--
	4th	On	On	A	--	O	A	A	--	O	A	O	--
A = Applied H = Holding * = Holding but not effective O = Overrunning

Circuit Description

The Transmission Control Module (TCM) calculates gear ratio from the Automatic Transmission Input Shaft Speed (A/T ISS) Sensor and Output Shaft Speed (OSS) Sensor readings. The TCM compares the known transmission gear ratio to the calculated gear ratio for the selected gear range.

When the TCM detects an unknown transmission gear ratio, DTC 85 sets.

Conditions for Setting the DTC

•	No DTC(s) 21, 22, 24, 28, 71, 72 and 87.

•	TP is greater than 25%.

•	Not in P/N or 4th gear commanded gear.

•	Engine speed is greater than 300 RPM.

•	Vehicle speed is greater than 7 mph.

•	Transmission Fluid Temperature (TFT) is greater than 24.5°C (76°F).

•	All conditions are met for 2 seconds.

Action Taken When the DTC Sets

•	The TCM does not illuminate the Malfunction Indicator Lamp (MIL).

•	The TCM will command maximum line pressure.

•	The TCM will inhibit TCC operation.

•	DTC 85 is stored in TCM history.

Conditions for Clearing the DTC

•	The TCM clears the DTC from Current to History when the fault condition(s) no longer exist and the ignition switch is cycled OFF and then ON.

•	A scan tool can clear the DTC from Current and History status. The TCM clears the DTC from the TCM history if the vehicle completes 40 ignition cycles without a failure reported.

•	The TCM cancels the DTC default actions when the fault no longer exists and the ignition is cycled OFF and then ON.

The fault condition(s) no longer exist, and the ignition switch is cycled OFF long enough to power down the TCM, and then ON.

Diagnostic Aids

•	DTC 85 will set when an unknown gear ratio is detected for any gear but 4th gear.

•	First diagnose and clear any engine DTCs or TP Sensor codes that are present. Then inspect for any transmission DTCs that may have reset.

•	Sticking or contaminated shift valves may cause an undefined gear ratio.

•	Refer to the Gear Ratio table.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

This step tests the indicated range signal to the actual selected range. A faulty TFP Val. Position Sw. could set DTC 28.

This step tests for proper ratios in each commanded gear state.

DTC 85

Step

Action

Value(s)

Yes

Was the Powertrain On-Board Diagnostic (OBD) System Check performed?

Go to Step 2

Go to On-Board Diagnostic System Check

Perform the transmission fluid checking procedure. Refer to Transmission Fluid Check .

Is the procedure complete?

Go to Step 3

Go to Transmission Fluid Check

Install the Scan Tool .
With the engine OFF, turn the ignition switch to the RUN position.

Important:: Before clearing the DTC's, record the Current and History DTC's information for reference. The Clear Info function erases the stored Current and History DTC from the TCM.

Record Current and History DTC's, then clear the DTC.
Start the engine.
Apply the parking brake, and select each transmission range: D 1, D 2, D 3, D 4, N, R and P.

Does the Scan Tool TFP Switch A/B/C display match each selected gear range? (Refer to Range Signal ).

Go to Step 4

Refer to DTC 28 TFP Manual Valve Position Switch Fault

Drive the vehicle in D 3 with Throttle Position (TP) greater than 25%.
Hold the vehicle speed above 11 km/h (7 mph) for greater than 2 seconds in each gear range.
Use the snapshot mode in order to record the transmission gear ratio for each commanded gear range: 1st, 2nd and 3rd.

Are the gear ratios within the specified range for each gear? (Refer to Shift Solenoid Valve State and Gear Ratio ).

Refer to Diagnostic Aids

Go to Step 5

Connect the Pressure Gauge to the transmission line pressure tap.
Perform the line pressure test. Refer to Line Pressure Check .

Is the line pressure within specifications for each selected gear range?

Go to Step 6

Go to Step 8

Remove the transmission oil pan. Refer to Transmission Oil Pan Removal Procedure, in the 4L80-E On-Vehicle Service.
Inspect the oil pan and the fluid for contamination.

Did you find excessive contamination?

Go to 4L80-E Unit Repair

Go to Step 7

Inspect the transmission for the following problems:

•	Sticking shift valves

•	Stuck apply pistons.

Did you find the condition?

Go to Step 10

Go to 4L80-E Unit Repair

Is the system line pressure low only in the specific gear which indicated an incorrect gear ratio?

Go to Step 9

Go to System Diagnosis Table; Low Line Pressure

Inspect for fluid pressure loss in the following areas:

•	1-2 SS Valve seal

•	2-3 SS Valve seal

•	Valve body passages

•	Valve body gaskets

•	Band apply pistons and seals

•	Clutch apply pistons and seals.

Did you find the condition?

Go to Step 10

Go to 4L80-E Unit Repair

In order to verify your repair, perform the following procedure:

Select DTC.
Select Clear Info.
Operate the vehicle under the following conditions:

•	Drive the vehicle in Reverse, 1st, 2nd and 3rd gear.

•	The TCM must see a valid gear ratio for the commanded gear for 10 seconds.

Are the gear ratios within the specified range for each of the selected gears?

1st: 2.38-2.63

2nd: 1.43-1.58

3rd: 0.95-1.05

Rev: 1.97-2.17

System OK

Begin Diagnosis again.

Go to Step 1